Amorphous carbon films have been deposited by ion beam assisted techniques and characterized by high resolution transmission electron microscopy, electron energy loss spectroscopy, and positron annihilation spectroscopy. It was found that the momentum transfer of bombardment has a strong influence on coating properties such as density, void concentration and sp3 content. The results can be explained by an ion pinning effect at low ion energies and a damage-induced graphitization at high ion energies.